This application relates generally to the protection of photovoltaic cells from reverse bias damage.
More specifically, this application relates to protecting the photovoltaic cells of a photovoltaic (PV) array from reverse bias damage by utilizing a rechargeable battery for bypassing current from a shaded photovoltaic cell or group of cells. Further, the invention mitigates the voltage degradation of a PV array caused by shaded cells.
Photovoltaic (PV) arrays, also known as solar arrays, are typically comprised of a plurality of photovoltaic cells (also known as solar cells) arranged in series in order to increase the voltage level of a PV array to a more usable amount, typically to 28-30 volts or even 120 volts. A plurality of series connected PV cells can then be connected in parallel to increase the current (and power) capability of the PV array. PV arrays are used extensively for terrestrial, orbital, and extra-terrestrial (e.g., planetary or interplanetary) uses.
These individual photovoltaic cells are typically constructed of a crystalline or amorphous silicon, or some other semiconductor material, such as the commonly used Gallium Arsenide (GaAs). When exposed to sunlight, these PV cells typically generate a voltage ranging from 0.50 to 2.5 volts each, depending on the materials used. The voltage of the PV portion of the device is determined by the nature of the p-n junction of the photovoltaic cell, or, in other words, the materials used. In the case of a GaAs homo-junction device this will be around 1.0 V. For thin-film a-Si or CuInSe2 (CIS) PV, the voltage generated will be somewhat less (0.4-0.8 V). Accordingly, strings of 30 or more photovoltaic cells are typically strung in a series to form a solar array in order to gain the desired output voltage.
Individual arrays of a series connected plurality of PV cells may then be placed in parallel in order to increase the total current and power capacity of the resulting entire PV array. A multiplicity of such arrays could further be combined to increase the power availability even more. The voltage output of individual arrays or the combination of arrays can then be modified using a DC-to-DC converters and/or a DC-to-AC inverters to generate a voltage useful for the typical electrical loads to be powered.
However, a problem arises when individual cells of the series connected photovoltaic cells are not generating electricity, such as when some subset of cells is shaded, for example. Because the current through series connected PV cells must pass through each cell in the series, if one or more individual PV cells are shaded, the current generated by the unshaded cells in the solar array must pass through the shaded cells as well.
This current through the shaded cell(s) results in a reverse bias across the cell, and can lead to “hot-spot” heating, which can damage the shaded cell. This problem is well-known in the art, and is also called “reverse-bias degradation”, “breakdown”, “shading”, and “shadowing” effects, for example. In the extreme, such “hot-spot” heating can destroy a photovoltaic cell, and thus degrade the array, or make it useless.
FIG. 1 shows a graphical example of such “hot-spot” heating, with curve 14 showing the operating points of 30 unshadowed cells (with point 14 representing the operating point with a partially shadowed cell) and with curve 11 representing the single, partially shadowed cell, operating far at the reverse bias point 12. Line Z represents a constant current line, and line H the nominal operating voltage. Quadrant A represents a reverse-bias, power dissipating area whereas quadrant B represents the power generating forward bias area.
Most localized shadowing, however, is transient, lasting only seconds or minutes. Shadowing of the entire solar array is not relevant to the above problem, because only partial or uneven shadowing leads to the “hot-spot” heating effect.
Conventional approaches for protecting the individual cells of a solar array include putting a “bypass diode” in parallel with each photovoltaic cell. FIG. 2 shows such an implementation. The bypass diode then shunts the series current of the solar array from the one or more cells that are shaded, protecting the shaded cells from damage.
Nevertheless, there are undesirable side-effects to this traditional approach. For example, the entire solar array loses operating voltage whenever one or more cells is shadowed. The amount of this voltage degradation is determined from the voltage no longer generated by the individual shaded cell(s), plus the turn-on voltage of the corresponding bypass diode(s), typically leading to a net voltage drop across the shaded cell, in contrast to the typical voltage rise of a voltage generating, unshaded cell. If the voltage of the solar array drops below the required bus voltage of the solar array, the entire array may not produce useful power. In practice, a shadow of as little as one percent might block one-hundred percent of the solar array output.
Accordingly, an approach that can overcome the above identified shortcomings would be desirable.
Further, it would be useful to utilized thin-film manufacturing processes for implementing the invention. Thin-film photovoltaic (TFPV) power generation has been under development for some time. TFPV sample cells and panels have flown in space. The principle benefits of TFPV arrays include very high mass specific power (W/kg), radiation tolerance and good stowability. The mission benefits of TFPV solar arrays have been identified, and may be realized when full scale TFPV arrays are constructed and space qualified.
In comparison to TFPV power generation, thin-film energy storage (TFES) is a relatively recent development. Very small thin-film lithium-ion batteries have been developed and tested in the lab for use in multi-chip modules (MCMs). With a typical operating range between 3.0 V and 4.2 V, the useable capacity of these initial TFES batteries is very small, ranging from 0.2 to 10 mAh/cm2. The energy capacities of thin-film batteries are typically too low to allow thin-film batteries to serve as primary energy storage for an array, but, can prove useful to solving some of the problems identified above.
Because of the similarity in the materials and processes that go into TFPV and TFES devices, it is practical to consider a combination of the two technologies. Further, a solution that in addition to providing protection against hot-spot heating, also enables some energy storage capability for momentary shading of the entire array, would add desirable additional benefit to the design.